1. Field of the Invention
The present invention is related to an image forming method in a copying machine, printer, facsimile machine, etc. and an apparatus using the method, particularly to a color image forming method of tandem type and an apparatus using the method.
2. Description of the Related Art
Generally, there is a full-color image forming apparatus using an intermediate image transfer belt (intermediate transfer body) composed such that toner images are superimposed sequentially on the intermediate transfer belt (primary image transferring) to form a color image, then the color image on the intermediate image transfer belt is transferred to a recording medium(recording sheet paper, hereafter referred to as a sheet paper) at a secondary image transferring position to obtain a sheet paper with the color image formed thereon. This kind of color image forming apparatus is called an intermediate tandem type image forming apparatus. On the other hand, there is an apparatus composed such that toner images are transferred sequentially on a sheet paper transferred on a sheet transfer belt to obtain a sheet paper with a color image formed thereon. This kind of color image forming apparatus is called a direct tandem type image forming apparatus. Hereinafter, both the intermediate image transfer belt and the sheet transfer belt are generically referred to as the transfer belt.
In the tandem type image forming apparatuses, the toner adhered to the transfer belt is cleaned periodically. When cleaning, the toner remaining on the transfer belt is removed for example by a cleaning mechanism provided with a blade or fur brush. However, it may happen through the cleaning of the transfer belt by the blade or fur brush that scuff marks (scratches) are produced on the transfer belt and normal image transfer can not be performed resulting in a faulty image formation.
There has been disclosed an image forming apparatus composed such that each image of yellow, magenta, cyan, and black toner is formed on each photosensitive drum and each image is transferred sequentially to a sheet paper transferred on the transfer belt, wherein, when performing the cleaning to remove the toner remaining on the transfer belt by reversely transferring the toner remaining on the transfer belt to at least one of the photosensitive drums, the difference between the peripheral velocity of the photosensitive drum to which the toner remaining on the transfer belt is reversely transferred and the transfer velocity of the transfer belt is 3% or larger in order to prevent the occurrence of a problem like that mentioned above (see Japanese Laid-Open Patent Application No. 2001-117460, hereafter referred to as patent literature 1).
Further, there has been disclosed an image forming apparatus composed such that the electric sources for applying bias voltage to transfer the second color and third color (magenta and cyan) among the electric sources for applying transfer bias voltage are capable of applying bias voltage of both positive and negative polarity to transfer the toner remaining on the transfer belt reversely to the photosensitive drums, a first cleaning mode and second cleaning mode are provided, in the first cleaning mode the peripheral velocity of the photosensitive drum coinciding with the transfer velocity of the transfer belt, in the second cleaning mode both the velocities differing by 10%, and the first cleaning mode and second cleaning mode can be switched to each other depending on the amount of the toner remaining on the transfer belt(see Japanese Laid-Open Patent Application No. 2002-6577, hereafter referred to as patent literature 2).
However, in the image forming apparatus disclosed in patent literature 1 or 2, as the cleaning of the transfer belt is performed by allowing the peripheral velocity of the photosensitive drum to be different from the transfer velocity of the transfer belt, if foreign matter is transferred together with a sheet paper for example from the sheet tray of the apparatus and the foreign matter intrudes in the gap between the photosensitive drum and transfer belt, scratch may produced on the surface of the photosensitive drum and transfer belt depending on the shape and hardness of the foreign matter. If the scratch is produced, faulty image may be formed when transferring image.
Further, with a photosensitive drum using organic photoconductor (OPC), the life of the photosensitive drum is extremely reduced by severe abrasion of the surface layer of the drum if the peripheral velocity of the photosensitive drum differs from the transfer velocity of the transfer belt as mentioned above. In the case of a photosensitive drum using amorphous silicon (a-Si), surface abrasion can be reduced, however, even the a-Si photosensitive drum can not cope with the abrasion produced by foreign matter intrusion, and faulty image formation occurs.